Hydraulic control devices of the bleed-off type have long been widely used as hydraulic control devices for conventional work machines. With this type of control system, a directional control valve for controlling the flow of hydraulic fluid delivered from a hydraulic pump to a hydraulic actuator is equipped with a bleed-off channel that is arranged in a bypass line. Such a bleed-off type hydraulic system controls the flow rate of the hydraulic fluid supplied to the actuator by performing the bleed-off control of returning part of the delivery flow of the hydraulic pump to a tank via the bleed-off channel according to the operation amount (stroke) of the directional control valve.
For such bleed-off type hydraulic systems, technological development is being conducted to reduce the flow returned to the tank via the bleed-off channel (bleed-off flow) from the viewpoint of increasing the energy efficiency (see Patent Literature 1, for example).
In the hydraulic system described in the Patent Literature 1, the delivery flow rate of the hydraulic pump is controlled by a controller by using a control valve (directional control valve) of the closed center type. With this configuration, bleed-off control equivalent to that performed by the control valve (directional control valve) equipped with the bleed-off channel is reproduced without the need of actually releasing part of the delivery flow of the hydraulic pump to the tank.
A hydraulic control device for a work machine is generally equipped with a relief valve for the purpose of protecting the hydraulic equipment. When the delivery pressure of the hydraulic pump driving a hydraulic actuator is about to exceed a preset pressure of the relief valve, the relief valve operates to return part of the delivery flow of the hydraulic pump to the tank, by which the delivery pressure of the hydraulic pump is prevented from exceeding the preset pressure of the relief valve. However, even in this case, the relief flow returning from the relief valve to the tank leads to energy loss. Therefore, technological development for reducing the relief flow is being carried out (see Patent Literatures 2 and 3, for example).
In the hydraulic system described in the Patent Literature 2, a pump flow rate command value is calculated in each of positive pump flow rate control, pressure feedback control and PQ control, and the delivery flow rate of the hydraulic pump is controlled by selecting one of the pump flow rate command values that most reduces the pump flow rate. Here, the “pressure feedback control” means control that calculates the pump flow rate command value based on the deviation of the delivery pressure of the hydraulic pump from a pressure set value (cutoff pressure control). By this control, the relief flow (loss) is reduced and the energy efficiency is increased even when the delivery pressure of the hydraulic pump rises sharply (e.g., when the swing structure of a hydraulic excavator is driven).
In the hydraulic system described in the Patent Literature 3, when the pump flow rate command value for the pressure feedback control has been selected in the hydraulic system of the Patent Literature 2, flow rate increasing control of increasing the flow rate command value with the passage of time is performed from the time of selection. By this control, sufficient driving force (turning/swinging force and hill-climbing force on slopes) is secured by increasing the delivery pressure of the hydraulic pump in the latter half of the pressure feedback control.